The present invention relates to a method for producing polycarbonates by ester interchange, and more particularly to a method for producing an aromatic polycarbonate of high molecular weight, improved color tone, and low mold contamination during molding by means of an ester interchange reaction involving an aromatic dihydroxy compound and a carbonic acid diester compound.
In recent years, aromatic polycarbonates have come to be widely used in a variety of fields as engineering plastics having excellent impact resistance and other mechanical characteristics, and possessing superior heat resistance, transparency, and the like.
A so-called phosgene process, in which bisphenols and other aromatic dihydroxy compounds are caused to react with phosgene by interfacial polycondensation, is commercialized as a method for producing aromatic polycarbonates. The phosgene technique currently implemented on a commercial scale has a number of drawbacks, however. Examples of these drawbacks include the need to use extremely toxic phosgene, to process large amounts of sodium chloride byproducts, and to prevent air pollution issues and which may be brought about by the methylene chloride commonly used as a reaction solvent.
A method (melt process) for subjecting aromatic dihydroxy compounds and carbonic acid diesters to ester interchange reactions in which sodium hydroxide and other alkali metal salts are used as catalysts is known as a process for producing aromatic polycarbonates by a technique other than a phosgene process. This method is advantageous in that aromatic polycarbonates can be produced inexpensively, and has recently attracted particular attention because it dispenses with the use of phosgene, methylene chloride, or other toxic materials, and is hence preferred.
To produce a high-molecular-weight polycarbonate having excellent mechanical characteristics by such a melt process, it is necessary to distill out bisphenols, diphenyl carbonate, and other unreacted monomers from the high-viscosity polycarbonate melt. The resulting polycarbonate is therefore commonly exposed to high temperatures (250 to 330.degree. C.) in a high vacuum for extended periods of time. Sodium hydroxide and other alkali metal compounds, however, sometimes act as catalysts in ester interchange reactions, initiating decarboxylation reactions, Kolbe-Schmitt analog reactions, and other side reactions. These side reactions are disadvantageous in that the polycarbonate branches expressed by the formula shown below are produced, cross-linked products are obtained, the resulting polycarbonates are discolored, and difficulties are encountered during the production of the high-quality materials with balanced color tones and molecular weights commonly produced by the melt process ("Polycarbonate Resins," Nikkan Kogyo Shimbunsha Publishing House, Sep. 30,1969). ##STR1##
(where X is a straight or branched hydrocarbon group).
Various methods aimed at overcoming these shortcomings have been proposed. For example, JP (Kokai) 4-89824 discloses catalysts comprising (1) chlorine-containing basic compounds, (2) alkali metal compounds or alkaline-earth metal compounds, and (3) phosphoric acid or phosphoric acid esters; JP (Kokai) 4-46928 discloses catalysts comprising (1) electron-donating amine compounds and (2) alkali metal compounds or alkaline-earth metal compounds; and JP (Kokai) 4-175368 discloses a method in which melt polycondensation is performed in the presence of an alkaline compound catalyst, an acidic compound and an epoxy compound are then added to the resulting reaction product, and the material is then treated at a reduced pressure.
The aforementioned method, however, is not necessarily successful in overcoming problems such as polycarbonate discoloration and mold contamination during molding.
As a result of research conducted in view of this situation, the inventors perfected the present invention upon discovering that the discoloration, mold contamination, and other such problems are caused by Fe, Ti, Cr, and other transition metal components released by the reactors, piping, and other equipment used during the melt polycondensation of polycarbonates, and found that a polycarbonate having an improved color tone and causing less mold contamination can be efficiently produced by a method in which a transition metal scavenger is added together with a sulfonic acid ester compound to a reaction product obtained by the melt polycondensation of a dihydroxy compound and a carbonic acid diester in presence of a catalyst containing an alkali metal compound, and the material is then treated at a reduced pressure.
A goal of the present invention, which was perfected in view of the above-described condition of prior art, is to provide an efficient method of producing a polycarbonate that has an excellent color tone and causes less mold contamination during molding.